Rod displacement measurements by X-ray CT and its impact on thermal-hydraulics in tight-lattice rod bundle (Joint research)

Mitsutake, Toru*; Katsuyama, Kozo* ; Misawa, Takeharu; Nagamine, Tsuyoshi*; Kureta, Masatoshi*; Matsumoto, Shinichiro*; Akimoto, Hajime

In tight-lattice bundles with about 1mm gap between rods, a rod displacement might affect thermal-hydraulic characteristics. The inside-structure observation of the simulated seven-rod bundle of RMWR was made with the high-energy X-ray CT of JNC. The CT view assured that the rod position was almost the same as expected by design. In the heat transfer experiments, all thermocouples on the center rod showed almost simultaneous BT-induced temperature increase and on the same axial heights showed quite similar time-variation behaviors in the vapor cooling heat transfer regime. It showed that the effect of the geometrical asymmetry was small on the BT characteristics. The calculated critical power by subchannel analysis with the input of the CT measured rod position was smaller by about 5% than that with the designed rod position. It concluded that the error in the calculated critical power was attributable not to the asymmetry in the rod position, but to the models in the subchannel analysis code.